Bellmac 32

The Bellmac 32 was a microprocessor developed by Bell Labs' processor division in 1980, implemented using CMOS technology and was the first microprocessor that could move 32 bits in one clock cycle. The microprocessor contained 150,000 transistors, and improved on the speed of CMOS design by using "domino circuits". It was designed with the C programming language in mind. After its creation, an improved version was produced called the Bellmac 32A, then cancelled along with its successor, the "Hobbit" C-language Reduced Instruction Set Processor.

History

The Bellmac 32 processor was developed by AT&T engineers in three different Bell Labs locations, Indian Hill (Naperville, Illinois), Holmdel, New Jersey and Murray Hill, New Jersey.

As the designers did not have automation tools, every chip designer had to use colored pencils for the completion of the initial design. Later, Steve Law developed a computer program that aided in the digitization of the initial designs.

The development of the Bellmac 32 produced a novel circuit design technique called domino logic, deemed a breakthrough for the production of the microprocessor. Tests performed during manufacture indicated that a clock frequency even higher than the 4 MHz target speed was possible. Implementing the control logic, however, proved unexpectedly complicated. These complications limited the final speed, when the entire chip was finished and tested, to 2 MHz. The team considered it as progress, but not as successful, as it could not meet the initial AT&T design goals.

Followup design meetings resulted in the Bellmac 32A project, as a second generation of the Bellmac microprocessor. The project once again selected CMOS technology and fixed the target clock frequency at 6.2 MHz. Adjustments to maximize the size of transistors and resistors and minimizing interconnections were fundamental in meeting the specifications. The engineers placed a 20-foot-by-20-foot engineering drawing of the chip layout on the floor of a large room.[1] Testing of chips produced from the completed circuit exceeded the design speed, and reached clock frequencies of 7.8 MHz, and even 9 MHz.[2]

After the breakup of AT&T, Bell Labs became a component of Western Electric. With this change, the Bellmac 32 was renamed to WE 32000. Updated versions of the chip included the WE 32100 and WE 32200 processors.

Architecture

The Bellmac 32 has a pipelined architecture with an instruction fetch unit that serves to control access to main memory, and an execution unit which serves to monitor the process and manipulate data.

The instruction queue is filled with the instructions fetched from the memory. The address arithmetic unit serves for address calculations.

Bellmac 32 hardware was able to store all instructions, data and register contents associated with a process during a context switch.

Registers

Bellmac 32 has a program counter and 15 general-purpose 32-bit registers. Three of these are used to support the operating system and can be used when the microprocessor is in kernel mode. It has three other registers that are used by some instructions as stack pointers.

Instructions

This microprocessor has 169 instructions, which are optimized for executing programs written in the C programming language. Accordingly, the format of character strings is adapted to C language specifications, for example.

The instructions may have up to three operands. The processor has no floating-point and decimal arithmetic instructions, which were later provided by coprocessors.

Memory

The Bellmac 32 implements multiple types of memory addressing, such as linear, immediate 8, 16 or 32 bits, registration, register indirect, short shift, absolute and indirect displacement of 8, 16 or 32 bits.

References
  1. http://greg.org/archive/2011/12/09/on_the_bellmac-32_and_perhaps_the_worlds_largest_plotter_pen_drawing.html
  2. http://www.ieeeghn.org/wiki/index.php/First-Hand:The_AT%26T_BELLMAC-32_Microprocessor_Development
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